1. Field of the Invention
The present invention relates to a probe coil system, used in a magnetic resonance (MR) apparatus for obtaining anatomical or qualitative information of an object utilizing an MR phenomenon, for applying an electromagnetic wave signal for exciting an MR phenomenon in an object and/or detecting an MR signal from the object and, more particularly, to a probe coil system for an MR apparatus capable of transmitting an RF excitation signal corresponding to a plurality of types of resonance frequencies or detecting an RF resonance signal.
2. Description of the Related Art
The MR phenomenon is a phenomenon in which an atomic nucleus placed in a static magnetic field and having a spin or magnetic moment resonantly absorbs only an electromagnetic wave having a predetermined frequency. This atomic nucleus resonates at angular frequency .omega.0 (.omega.0=2.pi..nu.0, .nu.0: Larmor frequency) represented as follows: EQU .omega.0=.gamma.HO
where .gamma. is the specific gyromagnetic ratio of the specific atomic nucleus and HO is the static magnetic field intensity.
In such a system for diagnosing a living organism utilizing the MR phenomenon, the MR phenomenon is excited in an object, and an electromagnetic wave of the resonance frequency is induced after absorption of the resonance is received and processed, thereby obtaining information of, e.g., a tomographic image of, the object.
In this system, in principle, the MR phenomena can be excited in and MR signals can be acquired from all portions of the object. However, due to limitations of an apparatus and clinical demands for a diagnosis image, actual conventional apparatuses utilize a gradient magnetic field to perform excitation of MR and acquisition of the MR signal for a specific portion, e.g., a specific slice, of an object.
For example, as shown in FIG. 1, a conventional medical diagnostic MR imaging apparatus comprises bed 1, static magnetic field coil 2, gradient magnetic field generation coil 3, probe coil system 4, static field power supply 6, X-, Y-, and Z-gradient power supplies 7, 8, and 9, transmitter 10, receiver 11, sequencer 12, and control processor 13. Bed 1 includes movable board 1a on which object P is placed. Static magnetic field coil 2 is driven by power supply 6 and generates a static magnetic field. Gradient magnetic field generation coil 3 is driven by power supplies 7, 8, and 9 and generates X-, Y-, and Z-gradient magnetic fields, respectively. Probe coil system 4 comprises at least one coil including a transmitting coil and a receiving coil or a transmitting/receiving coil for both transmission and reception. System 4 is driven by transmitter 10 and transmits a rotational magnetic field which is an RF signal for exciting MR. The MR signal induced in the object is detected by receiver 11 through system 4. Sequencer 12 drives and controls power supplies 7, 8, and 9 and transmitter 10 in accordance with a predetermined pulse sequence. Control processor 13 controls operations of bed 1 and sequencer 12 and processes the MR signal detected by receiver 11. Processor 13 includes a display and outputs a result of signal processing, e.g., displays the result on the display.
This system is used as follows.
Object P is placed on board 1a of bed 1, and board 1a is moved so that object P is located in a static magnetic field generated by static magnetic field coil 2. Then, transmitter 10 is driven by sequencer 12 in accordance with the predetermined sequence and causes probe coil system 4 to transmit, e.g., a 90.degree. or 180.degree. pulse as a rotational magnetic field, i.e., an excitation pulse for exciting MR. At the same time, power supplies 7, 8, and 9 are driven to cause gradient magnetic field generation coil 3 to apply a gradient magnetic field to object P.
Upon application of the excitation pulse and the gradient magnetic field, an MR phenomenon is generated in at least a predetermined portion of object P, and an induced MR signal is detected by system 4. The MR signal is fetched by control processor 13 and subjected to image processing such as image reconstruction processing. As a result, imaging information such as a tomographic image is obtained and displayed.
System 4 will be described below.
In order to obtain anatomical information of a living organism such as a slice image and qualitative information such as a spectroscopy using the above apparatus, a plurality of nuclear species are used or the static magnetic field is varied (e.g., an apparatus using a rampable magnet capable of enhancing and reducing the static magnetic field intensity in a short time period is used for spectroscopy). In this case, an RF signal of a resonance frequency applied from system 4 to object P or detected from object P differs in accordance with the type of atomic nucleus or with the static magnetic field intensity even if the atomic nucleus is not changed.
Examples are 1H, 21.3 MHz at 0.5 T, 42.6 MHz at 1 T, and 64 MHz at 5 T; 31p, 8.6 MHz at 0.5 T, 17.2 MHz at 1 T, and 25.8 MHz at 1.5 T; and 13C, 5.4 MHz at 0.5 T, 10.7 MHz at 1 T, and 16.1 MHz at 1.5 T.
In this case, a tuning frequency of a conventional probe coil system 4 is unconditionally determined in accordance with an inductance of the coil. For this reason, in order to use a plurality of nuclear species and to vary the static magnetic field, the tuning frequency of system 4 must be variably controlled.
According to an abstract "R.F. Coil Design for NMR Imaging (J. F. Shen and I. J. Lowe)" of the "Society of Magnetic Resonance in Medicine (Fourth Annual Meeting, Aug. 19-23, 1985)", a tuning frequency can be changed by inserting a shortening capacitor in a circuit system including a coil.
A probe coil system in which a shortening capacitor is inserted will be described below.
FIG. 2 is a circuit diagram showing coil L consisting of a plurality of coil elements of the probe coil system. FIG. 3 shows a circuit in which shortening capacitors each consisting of a plurality of capacitance elements are inserted between a plurality of coil elements of coil L similar to that shown in FIG. 2. In this case, assuming that the self resonance frequency of the circuit shown in FIG. 2 is fself, the self resonance frequency fself' obtained when shortening capacitor Cs consisting of a plurality of capacitor elements is inserted as shown in FIG. 3 is represented as follows: EQU fself'&gt;fself
FIG. 4 shows an equivalent circuit of a probe coil system obtained by connecting, in a circuit mainly consisting of the coil shown in FIG. 2, tuning capacitor C1 in parallel with the coil and matching capacitor C2 in series therewith.
In FIG. 4, reference symbol L0 dentes an inductance of the coil; r0, an equivalent resistance caused by the coil itself and object P inserted therein; and Z0, an output impedance of the probe coil system which is set to coincide with a characteristic impedance of a cable connected to the probe coil system. If a circuit including the shortening capacitor shown in FIG. 3 is a main element of a circuit system of this probe coil system, shortening capacitor Cs is a capacitance connected in series with L0 and Z0 as indicated by broken lines representing Cs.
In this manner, a tuning frequency can be changed by inserting shortening capacitor Cs in a circuit system including the coil of the probe coil system. However, when shortening capacitor Cs is inserted, the equivalent resistance r0 and output impedance Z0 are changed. Therefore, tuning capacitor C1 and matching capacitor C2 must be adjusted.
For this reason, if shortening capacitor Cs inserted in the coil portion is a continuously variable capacitor and the capacitance of shortening capacitor Cs is changed to switch the tuning frequency to a plurality of different frequencies, the number of portions of the system to be adjusted is significantly increased. Therefore, the frequency cannot be practically tuned to a plurality of tuning frequencies. For this reason, in practice, a circuit in which a shortening capacitor is not inserted as shown in FIG. 2 and a circuit in which a suitable shortening capacitor is inserted as shown in FIG. 3 are independently used. That is, the tuning frequency is fixed in the conventional apparatus.
The tuning frequency can be varied by switching a circuit in which the shortening capacitor is not inserted and a circuit in which the shortening capacitor is inserted. In this case, however, capacitors C1 and C2, which constitute a matching circuit, can be used as they are only when the shortening capacitor has a particular capacitance, which is very rare in practice. As a result, the arrangement is complicated, and only two tuning frequencies can be set. Therefore, according to the conventional techniques, the MR signals cannot be acquired using a plurality of nuclear species or varying a static magnetic field.